A growing understanding of the role microRNAs (miRNAs) play in brain development and function and in synaptic plasticity, combined with the recognition that a single miRNA can regulate the expression of multiple genes across networks of biochemical pathways, is fueling interest in miRNAs as targets for intervening in psychiatric disorders.
At the recent Society for Neuroscience annual meeting in Washington, D.C., researchers presented studies evaluating the association between miRNA dysregulation, neurodevelopment, schizophrenia risk, and anxiety disorders.
Schizophrenia and bipolar disorder each affect about 1% of the population in the U.S., costing more than $100 billion/year and taking a high toll on families of affected individuals.
Although both disorders tend to have high heritability, they are not strictly genetic diseases and disease development may depend on a complex interplay between genes and environmental triggers. Further complicating the picture, both diseases involve dysregulation of multiple signaling pathways; additionally, different molecular aberrations can result in a similar phenotype.
The predominant mechanism by which a miRNA silences a target gene in mammals is through post-transcriptional regulation, preventing a strand of messenger RNA (mRNA) from being translated into a protein.
Claes Wahlestedt, M.D., Ph.D., professor of psychiatry and behavioral sciences, Miller School of Medicine, University of Miami, described miRNAs as “master regulators” due to the ability of a single miRNA to target hundreds of mRNAs and the emerging consensus that miRNAs regulate more than half of all protein-coding genes.
This regulatory layer “may account for some of the missing genetic/epigenetic variability in the etiology of psychiatric disease,” he said.
At present, the most significant known risk factors for schizophrenia are copy number variations (CNVs), with the 22q11 deletion being the most common. It is present in about 1 in 4,000 people and in 1% of people with schizophrenia.
Most cases of 22q11 deletion syndrome (DS) involve a de novo deletion, typically about 3 million base pairs in size, and are not inherited. The disorder has variable presentations and individuals carrying the deletion have about a 25% risk of developing schizophrenia.
Included in the 22q11 deletion is the DGCR8 gene, which has a key role in the biogenesis and maturation of miRNAs to their mature, shortened form. Linda Brzustowicz, M.D., professor, department of genetics, Rutgers University, introduced the concept of canalization, which refers to the robustness of a trait or phenotype and the ability of a phenotype to be expressed regardless of genotypic or environmental variability. She proposed that “miRNAs may be a mechanism of canalization.”
The fact that schizophrenia does not develop in about 75% of people with 22q11-DS may be attributable to the increased capacity of canalized traits to absorb mutational variance. Dr. Brzustowicz presented a model in which 22q11-DS results in DGCR8 haplotype insufficiency, which perturbs the miRNA regulatory system, allowing previously silenced regulatory mutations to alter gene expression. DGCR8 mutations result in a reduction in one or more subsets of miRNA, with the consequence of increased gene expression.
By determining which miRNAs are reduced in the brains of patients with 22q11-DS and applying the canalization hypothesis, it may be possible to predict which genes are likely to have increased expression and to have a role in the development of schizophrenia.
Maria Karayiorgou, M.D., professor of psychiatry at Columbia University, is also studying 22q11 mutations, and specifically the 22q11.2 microdeletion.
Dr. Karayiorgou uses a mouse model of 22q11 CNVs, characterized by deletions on mouse chromosome 16 that correlate to 22q11 CNVs in humans. She applies behavioral assays to study cognitive deficits and has reported deficits in spatial working memory tasks indicative of decreased working memory capacity. These behavioral deficits are associated with DGCR8 deficiency in DGCR8 knockout mice.
The 22q11.2 microdeletion does not appear to affect basal synaptic transmission in mice, but it may affect the activity of neuronal networks in the prefrontal cortex. The mutated mice exhibited reduced synaptic memory in this region of the brain. Dr. Karayiorgou concluded that miRNA dysregulation likely contributes to the cognitive impairment seen in the mouse model by altering short-term synaptic plasticity.
A presentation by Brooke Miller, Ph.D., research associate at Scripps Research Institute, demonstrated that “miR-132 dysregulation in schizophrenia has adult and neurodevelopmental implications.” She described the analysis of human brain tissue samples from 34 control subjects, 35 patients with schizophrenia, and 31 individuals with bipolar disease, evaluating each for 854 miRNAs using microarrays. Only 2 of the 854 miRNAs were differentially expressed in schizophrenia and 10 in bipolar disease; miR-132 overlapped both.
Dr. Miller gave several reasons why miR-132 is of particular interest: its transcription is directly regulated by the cAMP-response element binding protein; it regulates synaptic outgrowth; it potentiates N-methyl-d-aspartate (NMDA) receptor signaling; and its targets are significantly upregulated in tissue samples from individuals with schizophrenia.
In fact, more than 25 (13%) of putative miR-132 targets are significantly upregulated in schizophrenia. miR-132 also has a role in regulating gene expression in mice during postnatal weeks 2 to 4, which corresponds to adolescence in mice, a crucial period of neurodevelopment.
“Of 201 miR-132 targets, 57 overlap with gene-expression changes in that developmental period,” she said.
Stephen Magill, an M.D., Ph.D. student at Oregon Health and Science University, provided evidence to support the hypothesis that “miR-132 regulates dendritic growth and arborization of adult newborn hippocampal neurons.” He studied the role of miR-132 at the cellular level.
In mouse studies, using sensors of miRNA activity that have single-cell resolution, Magill and colleagues showed that ablation of miR-212/132 resulted in significantly decreased dendrite outgrowth in vivo, including reductions in both the length and branching of dendrites. miR-132 is the predominant functional product of miR-132/212.
Ilris Hovatta, Ph.D., University of Helsinki, looked at differences between inbred mouse strains as a means of studying mRNA and miRNA networks and defining mechanisms that regulate genes associated with anxiety disorders. Dr. Hovatta presented data on miRNA expression in various brain regions, including the prefrontal cortex, hippocampus, and hypothalamus, and identified anticorrelations between miRNA and mRNA pathways. As expression of a miRNA increases, expression of its target mRNA decreases.
An unbiased genome-wide screen led to the identification of 69 miRNAs with expression levels that correlated with anxiety-like behavior. The researchers used pathway analysis to predict their target mRNAs and the gene-regulatory networks that control anxiety in mice.
Schahram Akbarian, M.D., Ph.D., associate professor of psychiatry, University of Massachusetts Medical School, described “Neural epigenomes in developing and diseased prefrontal cortex.”
Epigenomic mapping studies, with a focus on hydroxymethylation of cytosine, have identified more than 100 site-specific covalent modifications affecting nucleosome core histone proteins that are important for normal brain development. As part of the Brain Epigenome Project at UMass Medical School, researchers have studied more than 80 billion base pairs of DNA from nucleosomes derived from neurons in the prefrontal cortex. A comparison of 15 epigenomes identified differences between neuronal and non-neuronal epigenomic signatures.